[RFC 2/2] ASoC: rt5670: Add LED trigger support

[Jaroslav Kysela]

Dne 24. 02. 21 v 8:12 Takashi Iwai napsal(a):
> On Tue, 23 Feb 2021 21:56:16 +0100,
> Jaroslav Kysela wrote:
>>
>> Dne 23. 02. 21 v 17:20 Takashi Iwai napsal(a):
>>>>> Of course, this implementation would make the integration much easier,
>>>>> and that's a big benefit.  So I have a mixed feeling and not decided
>>>>> yet whether we should go for it right now...
>>>>
>>>> I think that we can reconsider the LED handling implementation later, when
>>>> someone brings something better on the table.
>>>
>>> What worried me is the plan to expose this capability to user-space.
>>> If it's only a kernel-internal, we can fix it in the kernel and
>>> nothing else broken, but if it's a part of API, that's not easy.
>>>
>>> So, if any, I'd like to avoid exposing to the user-space at first.
>>> (But then it comes to the question how to deal with a case like AMD
>>> ACP...)
>>
>> I tried to propose a complete solution and the ACP was one strong reason for
>> this kernel / user space API. So without the user space support, it's just
>> a half solution for known issues.
>>
>> Frankly, I don't see any drawback or a problem even if we remove this API
>> later.
> 
> Removing the user-space API is absolutely no-go.  The only exception
> would be either the case really no one uses it or it's too buggy and
> unfixable.

This is a special case. Even if those LED bits are ignored by kernel in
future, we expect to be replaced with another layer. Thus the functionality
must be retained.

>> The LED group bits are just informal for the user space and it's
>> expected to create the user controls tied to this LED functionality only in
>> alsa-lib/plugins at the moment. The kernel may return an error when the user
>> space tries to set those new bits when the API is deprecated and I believe
>> that the hardware design faults like AMD ACP (without the hardware mute) are rare.
> 
> The experience tells us that users are creative enough to (ab)use a
> new ABI in any unexpected ways, and we have no control for it.  So
> it's not about how alsa-lib is implemented but rather how ABI could be
> abused :)

Ok, I don't have other ideas. I don't agree with your argumentation for this
particular case, where the functionality is marginal. Ideally, the AMD driver
may be recoded to use double-buffering and software mute switch, so we should
handle everything in the kernel space.

					Jaroslav

-- 
Jaroslav Kysela <perex at perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.